1. Technical Field
The present invention relates to a lateral electric field liquid crystal display panel and a method for manufacturing the same. In more particular, the invention relates to a lateral electric field liquid crystal display panel designed so that a display image is not disturbed even when static electricity enters and to a method for manufacturing the above liquid crystal display panel.
2. Related Art
Since the thickness, the weight, and the size of a liquid crystal display panel can be advantageously reduced, the liquid crystal display panel has been widely used as display devices for various types of electronic apparatuses, such as a television, a personal computer, a mobile phone, and other mobile information terminals. This liquid crystal display panel has the structure in which a pair of substrates made of glass or the like is provided, the substrates are adhered to each other with a sealing material disposed in a non-display region between the pair of substrates so as to form a predetermined space therebetween, liquid crystal is injected in the space through a liquid crystal injection port formed at a position to which the sealing material is not applied, and the injection port is sealed after the liquid crystal is injected.
This liquid crystal display panel is formed such that first and second electrodes are provided on the above two substrates or one substrate thereof, and a predetermined voltage is applied between the electrodes so as to display various images in a display region. As this type of liquid crystal display panel, there are a display panel in which the first and the second electrodes are separately disposed on the different substrates and a predetermined voltage is applied between the electrodes, and a display panel in which the first and the second electrodes are provided on only one of the pair of electrodes and a predetermined voltage is applied between the electrodes. Among those described above, the former is called a vertical electric field liquid crystal display panel, and the latter is called a lateral electric field liquid crystal display panel.
As the vertical electric field liquid crystal display panel, for example, a twisted nematic (TN) mode, a vertical alignment (VA) mode, and a multidomain vertical alignment (MVA) mode, each having its own features, may be mentioned. In addition, as the lateral electric field liquid crystal display panel, for example, an in-plane switching (IPS) mode and a fringe field switching (FFS) mode, each having its own features as with the above, may be mentioned.
In particular, compared to the vertical electric field liquid crystal display panel, the lateral electric field liquid crystal display panel can advantageously increase a viewing angle; hence, in recent years, panels using this mode have been increasingly used. However, in the lateral electric field liquid crystal display panel, a problem in that an image is disturbed due to the influence of static electricity occurs, although this problem has not occurred in the vertical electric field liquid crystal display panel. That is, since the vertical electric field liquid crystal display panel has the structure in which electrodes are disposed on the respective first and the second substrates, and a liquid crystal layer is formed therebetween, the electrodes disposed on the substrates each have an electrostatic shielding function, and even when static electricity tries to enter from the outside, it is shielded by these electrodes, so that an image is not adversely influenced.
On the other hand, in the lateral electric field liquid crystal display panel, since the first and the second electrodes are both provided only one of a pair of the first and the second substrates and are not provided on the other substrate, the other substrate has no electrostatic shielding function. Hence, when static electricity enters from the side of the substrate which is provided with no electrodes, an image is adversely influenced.
Hence, a liquid crystal display device incorporating a lateral electric field liquid crystal display panel has been proposed in which a shielding conductive layer is provided on a substrate as an electrostatic countermeasure (for example, see JP-A-9-105918, paragraphs [0018] to [0025] and [0071], FIG. 1). With reference to FIG. 12, a liquid crystal display device disclosed in JP-A-9-105918 will be described. FIG. 12 is a cross-sectional view of an important portion of the liquid crystal display device of the related art described in JP-A-9-105918.
A liquid crystal display device 30 has a liquid crystal display panel composed of a liquid crystal layer LC provided between an upper substrate 31A and a lower substrate 31B, and a backlight unit B/L disposed at the lower substrate 31B side. The lower substrate 31B includes a polarizer 32 at an exterior surface side, and the upper substrate 31A includes a glass substrate 35A, a translucent shielding conductive layer 33, and a polarizer 34 in that order from the bottom shown in the figure.
In each pixel of the upper substrate 31A, a reference signal line and a scanning signal line are disposed parallel to each other with a relatively large space therebetween, and a plurality of reference electrodes is integrally formed with the reference signal line and is covered with an insulating film. In addition, a thin film transistor is provided on this insulating film. A color filter is formed in the lower substrate 31B. The shielding conductive layer 33 is formed of a film primarily composed of indium tin oxide (ITO), SnO2, or In2O3.
According to the liquid crystal display panel disclosed in JP-A-9-105918, since the shielding conductive layer 33 is included in the upper substrate 31A located further from the backlight unit, static electricity is electrostatic-shielded by this shielding conductive layer 33, and hence, an image is prevented from being disturbed. This shielding conductive layer 33 is formed of a film primarily composed of ITO, SnO2, or In2O3 However, when this shielding conductive layer 33 is formed, for example, of an ITO film among an ITO, a SnO2, and an In2O3 film, the ITO film thus formed may disappear without being recognized, and as a result, the shielding effect may not be obtained in some cases. Just for reference, the ITO film has been very commonly used as a transparent conductive film forming a pixel electrode and a common electrode of a liquid crystal display panel. Accordingly, when the shielding conductive layer composed of a transparent conductive film is formed using an ITO film, various advantages, such as efficient material usage and easy handling, are obtained; hence, the ITO film is generally used for the shielding conductive layer in many cases, On the other hand, JP-A-9-105918 has disclosed that materials other than ITO are described for the shielding conductive layer; however, for example, since the above materials other than ITO are actually very difficult to be etched as compared to ITO, or the etching may not be possibly carried out in practice, it is not known whether the materials other than ITO are actually used for a pixel electrode and a common electrode and further for the shielding conductive layer.
In addition, since this shielding conductive layer 33 is provided on the glass substrate 35A, the display quality may be degraded depending on film formation ways, that is, depending on the film thickness, the transmittance, the sheer resistance, and the like. For example, pits and/or scratches may be generated on a surface of this glass substrate during its manufacturing process, and when the pits and the like are covered with the shielding conductive layer 33, the pits are liable to be easily viewed due to the relationship with the reflectance; hence, the display quality may be degraded in some cases. However, in the liquid crystal display panel disclosed in the above document, JP-A-9-105918, the problem described above has not been taken into consideration at all.
In order to develop an electrostatic countermeasure for a lateral electric field liquid crystal display panel, the inventors of the invention carried out research on the shielding effect in such a way that the shielding conductive layer is formed using an ITO film, and a polarizer is adhered to this ITO film. However, the case occurred unexpectedly in that since the ITO film formed on a substrate disappeared without being recognized, the shielding effect could not be obtained. It was understood through the investigation of this case that since the ITO film formed on the substrate was an amorphous film, this amorphous ITO disappeared by a chemical reaction with a material forming the polarizer. Hereinafter, the reason this ITO film disappeared will be further described.
In recent years, the liquid crystal display panel has been requested to have a smaller thickness in accordance with its application. This reduction in thickness has been performed through the steps of: forming a pair of substrates; applying a sealing material composed of a thermosetting resin to a non-display region of one of the above two substrates; then adhering the two substrates to each other by applying heat to the sealing material thus applied for curing thereof; decreasing the thickness of the substrates adhered to each other by mechanical polishing, chemical etching, or the like. Subsequently, an ITO film is formed as the shielding conductive layer on the surface of one of the substrates; however, since this ITO film is formed after the two substrates are adhered to each other, the ITO film cannot be formed at a temperature or more at which the thermosetting resin which forms the sealing material is adversely influenced.
That is, when the ITO film is formed at a high temperature at which the sealing material is adversely influenced, the sealing material composed, for example, of a thermosetting resin may peel off. Hence, the ITO film used as the shielding conductive layer is formed at a low temperature. However, when the ITO film is formed at a low temperature, the ITO film thus formed is an amorphous film. As a result, when a polarizer is adhered on the amorphous film in a subsequent step, because of a material component forming this polarizer, the phenomenon occurs in which the ITO film thus formed disappears.
For example, as shown in FIG. 13, the polarizer 34 of the liquid crystal display panel disclosed in JP-A-9-105918 is formed such that, protective films 34a are adhered to two sides of a polyvinyl alcohol (PVA) film 34b which is obtained by stretching a polyvinyl alcohol polymer which generally contains iodine or an organic material such as a dye in a predetermined direction. The protective film 34a is formed of tri-acetyl-cellulose (hereinafter referred to as “TAC”), a thermoplastic saturated norbornene resin, or the like. In addition, for example, under high humidity and high temperature conditions, acid components contained in the organic material, TAC, and the like, which form the polarizer, ooze out (in the figure, outline arrows) and cause a chemical reaction with the amorphous ITO formed as the shielding conductive layer 33, so that the ITO is made to disappear.
In addition, as a method for preventing ITO used as the shielding conductive layer 33 from disappearing, it may be possible that before the substrates are adhered to each other, for example, a crystallized ITO film which is not likely to chemically react with an acid component may be formed in advance instead of the amorphous film. However, when the thickness of a liquid crystal display panel is decreased, since the thickness of the substrate is generally processed by polishing or etching after two substrates are adhered to each other, even when the ITO film as described above is formed in advance, the ITO film is polished away in the step of decreasing the thickness of the substrate.
Alternatively, it is also considered that in order to suppress the ITO film from disappearing, the thickness thereof used as a shielding conductive layer is increased; however, even in the case described above, the ITO film may completely disappear in some cases. In addition, when the thickness of the ITO film used as a shielding conductive layer is increased, since the refractive index of ITO is inherently high (n=1.8 to 2), and pits (irregularity defects) and scratches are generated on the surface of the substrate in the step of decreasing the thickness thereof, the pits and the like are liable to be easily viewed, so that a new problem of degradation in display quality may arise.
Accordingly, when the shielding conductive layer is formed using ITO, an amorphous ITO film is formed because of the relationship with a panel manufacturing process, and hence a polarizer composed of a material that will not react with this amorphous ITO must be selected. As a result, the selection of the polarizer is very limited, and hence the manufacturing cannot be easily carried out. Accordingly, in consideration of the situation described above, intensive research was carried out by the inventors of the invention. As a result, it was found that when a material which did not disappear even if it was in contact with a currently commercially available polarizer was selected for a shielding conductive layer instead of ITO, and when the thickness, the sheet resistance, and the transmittance of the above material were set in respective predetermined ranges, a liquid crystal display panel which caused no degradation in display quality could be obtained; hence, the invention was finally completed.